It is a common practice for vehicle manufacturers to calibrate vehicle speedometers such that they report a greater speed than that which the vehicle is actually traveling. This practice is sometimes called “positive error calibration.” This approach is taken to ensure that the actual vehicle speed is never under reported by the vehicle speedometer. Although the motivation for this approach is understandable, it renders a vehicle's speedometer ineffective as a device for accurately gauging estimated time of arrival.
Like the vehicle speedometer, the vehicle odometer also suffers from lack of precision. Although the vehicle odometer is calibrated to give generally acceptable average distance measurements, it is calibrated from the rotation of one or more vehicle drive train members which ultimately are dependant upon the rotation of one or more vehicle wheels. The wheel diameter parameter is a particularly weak link in accurately determining vehicle speed and distance traversed by the vehicle in part because of its diameter is susceptible to variance due to wear, internal air pressure, vehicle loading, manufacturing tolerances, temperature, and other such factors. Thus, even if vehicle speedometers and odometers were highly accurately calibrated at the time the vehicle was new the factors mentioned above, acting on the tire over time, would erode the accuracy of the speedometer and odometer.
Thus, there is a need for an inexpensive system for continually recalibrating a vehicle's speedometer and odometer over the lifetime of the vehicle.